Have you ever seen a liquid magnet? If magnetic material is ground into an extremely fine powder, with a particle size of about 10 nanometers (see Nanoguitar for more information on length units), and suspended in a liquid, the resulting magnetic suspension is called a ferrofluid.

Ferrofluid, with permanent magnets underneath (photo courtesy of Felice Frankel, MIT)

Magnetic particles, with surfactant molecules attached, suspended in a fluid to form a ferrofluid. The surfactant prevents clumping of the magnetic particles. (Drawing courtesy of Ferrotec Corporation [USA] ).

A ferrofluid forms spikes along the magnetic field lines when the magnetic surface force exceeds the stabilizing effects of fluid weight and surface tension (see photo). The photo shows a ferrofluid on a flat surface, with permanent magnets underneath. Since the magnetic particles in the ferrofluid attract each other, they must be kept apart, so they are coated with a soap-like material called a surfactant (see drawing).

The ferrofluid has low resistance to flow, so it can function as a liquid O-ring seal, kept in place with permanent magnets. Ferrofluid seals prevent dust particles from working into disk drive mechanisms and crashing the drive head into the disk. Also, ferrofluids cool the coils of high-power, hi-fi speakers.

A possible future medical application might be an injectable ferrofluid that could be concentrated at a point in the body with an external magnetic field. The ferrofluid could deliver medication or act as a contrast agent for magnetic resonance imaging.